Hitherto, a sampling rate converting apparatus formed into a digital filter structure has been used for the purpose of sampling an analog signal at a predetermined sampling frequency and converting a digital signal thus-obtained into an arbitrary sampling frequency.
In general, a sampling rate converting apparatus of the type described above is constituted by a high-order oversampling filter for the purpose of strictly securing the Nyquist frequency as the conversion characteristics of its transmission system.
In a case where, for example, the sampling frequency of a 625/50 component digital video signal formed in accordance with the D-1 format for a digital video tape recorder (DVTR) is, by using a sampling rate converting apparatus of the type described above, converted into a sampling frequency, which corresponds to a PAL composite digital video signal formed in accordance with the D-2 format, the sampling frequency cannot directly be converted between the digital video signals because the rate of the sampling frequency is converted from a frequency of 13.5 [MHz] into a frequency of 17.734475 [MHz]. Therefore, an oversampling filter having a length of about 16500 orders must be constituted for approximation.
On the contrary, when the sampling frequency of a PAL composite digital video signal is converted into a sampling frequency which corresponds to a 625/50 component digital video signal, an exclusive oversampling filter of the equivalent circuit size to that of the above-described structure must be constituted in order to convert the rate of the sampling frequency from a frequency of 17.734475 [MHz] to a frequency of 13.5 [MHz]. Therefore, each of the structures must include an exclusive circuit as a whole. As a result, there arises a problem in that the circuit structure becomes too complicated and the size becomes too large. Disclosure of the Invention
In view of the prior art, an object of the present invention is to provide a sampling rate converting apparatus capable of converting the sampling frequency of a digital signal composed of a first or a second sampling frequency into the second or the first sampling frequency, that is, capable of converting the sampling rate in two opposing directions.
In order to overcome the above-described problems, according to a first aspect of the present invention, a sampling rate converting apparatus 1 for converting a digital signal DIN which is sampled by a first or a second sampling frequency f1 or f2 into the second or the first sampling frequency f2 or f1, the frequency ratio of which holds a simple integral relationship, at the first or the second sampling frequency f1 or f2, the sampling rate converting apparatus comprising: oversampling filters 2A to 2K, 3A to 3L and 4A to 4K composed of FIR type digital filters the length of each of which corresponds to the least common multiple of the frequency ratio of the first and the second sampling frequencies f1 and f2, wherein the first or the second sampling frequency f1 or f2 of the digital signal DIN is converted into the second or the first sampling frequency f2 or f1.
According to a second aspect of the present invention, there is provided a sampling rate converting apparatus 10 for converting a digital signal SIN10 which is sampled by a first or a second sampling frequency into the second or the first sampling frequency, the sampling rate converting apparatus comprising: oversampling filters 11A, 11B, 11C and 11D composed of FIR type digital filters the length of each of which corresponds to the least common multiple of the frequency ratio of the first and the second sampling frequencies; coefficient generating means 17A, 17B, 17C, 17D and 17E including a plurality of selectable coefficient data items, and giving coefficients C20, C21, C22, C23 and C24, which corresponds to the selected coefficient data, to weighting means 15A, 15B, 15C, 15D and 15E of the oversampling filters 11A, 11B, 11C and 11D, wherein when the first or the second sampling frequency of the digital signal SIN10 is converted into the second or the first sampling frequency in a case where the frequency ratio of the first or the second sampling frequency does not hold a simple integral relationship, the coefficient data of the coefficient generating means 17A to 17E is selected in accordance with a direction of the conversion.
Furthermore, coefficients C20 to C24 to be given to weighting means 15A to 15E of oversampling filters 11A to 11D are changed by selecting coefficient data of coefficient generating means 17A to 17E including a plurality of selectable coefficient data so that, in a case where the frequency ratio of the first or the second sampling frequency does not hold a simple integral relationship, the first or the second sampling frequency of the digital signal SIN10 can be converted into the second or the first sampling frequency.